This invention relates generally to the field of extensometry or displacement measurement. More particularly, this invention relates to a new and improved x-ray based displacement measurement method and apparatus particularly well suited for hostile and/or high temperature environments.
Improved ability to produce engineering components for hostile environment use is very important for continued increases in efficiency of fuel burning engines, and is a prerequisite for successful development of the more ambitious hypersonic flight vehicles such as the National Aero Space Plane (NASP). A fundamental requirement of these and other advanced programs is the ability to measure the mechanical response of newly available materials under realistic operating conditions. These may include high-temperature - high-velocity gas flows, significant pressure gradients or the presence of flames and smoke. The measurement of strain and displacement under such conditions is very challenging and a limited number of available methods including high-temperature strain gages, ceramic rod extensometers, and laser optical systems can be used under specific environmental conditions. Strain gages seem to be limited to temperatures below 1000.degree. C. and strains of only a few thousand microstrain. Contacting extensometers present access problems in some cases and cannot be used in the presence of high-velocity gas flows. Laser based optical methods appear to be the least restrictive, but hot gases above ambient pressure are a severe problem because of refraction of the source laser beam. In addition, smoke and dust can cause the accuracy of such systems to greatly deteriorate.
A significantly different method and apparatus for extensometry which is particularly well suited for displacement measurement in hostile environments is based on x-rays (as opposed, for example, to prior art laser measurements). Such an x-ray based measurement apparatus has been described by the inventors herein in the following papers:
Jordan E. H.., Pease D. M., Canistraro H. A., Displacement Measurement Using a Scanning Focussed X-Ray Line Image, Advances in X-Ray Analysis: Proceedings of the 39th Annual Denver X-Ray Conference, Volume 34, Pergammon Press, New York, June 1991. PA1 Jordan E. H., Pease D. M., Canistraro H. A., X-Ray Beam Method for Displacement Measurements in Hostile Environments, Proceedings of the Sixth Annual Hostile Environments and Strain Measurements Conference - Society for Experimental Mechanics, Bethel Conn., November 1989.
The system described in the above papers is based on the ability to focus and scan low energy, hard x-rays such as those emanating from copper or molybdenum sources. The x-rays are focused into a narrow and intense line image which can be scanned onto targets that fluoresce secondary x-ray radiation. This radiation is monitored and target edge position can be determined by measuring the beam pointing angle when the marker begins to fluoresce. This system has the ability to conduct macroscopic strain measurement in hostile environments by utilizing two or more fluorescing targets. An important advantage of this technique lies in the penetrating nature of x-rays which are not affected by the presence of refracting gas layers, smoke, flame or intense thermal radiation, all of which could render conventional extensometry methods inoperative or greatly compromise their performance.
The prior art x-ray based measurement system comprised the components of FIG. 1 including an x-ray source 10 which emits x-rays to a bent crystal 12, a detector 14 for detecting secondary x-rays emitted from a fluorescing target 16 and a controller/processor (e.g., computer) 18 for controlling beam pointing angle and secondary count rate to the data acquisition system. In order to conduct the contemplated types of displacement measurements, beam stability and focus are critical, as any fluctuation in intensity can be misinterpreted as displacement. Because the focusing crystal only reflects for one incident angle, it is necessary to consider how the beam is to be scanned. This task was accomplished in the prior art by rotating the crystal (along an arc) on an arm whose center is located directly below the x-ray line source as shown in prior art FIG. 2. Testing of direct beam count rates revealed stability of better than 1 percent over a 1.25 cm linear range (approximated by an arc).
However, while well suited for its intended purposes, this prior art x-ray based system suffers from several important drawbacks and deficiencies. For example, the prior art method of scanning the x-ray image (along an arc) introduced several problems to the measurement technique. First the rotational arm only permitted a limited image scan range, greatly reducing the measurement gage length that could be examined by the device. Additionally, the rotational arm caused the image to move on the arc of circle, whereas the device is intended for uni-axial measurements, and this angular motion is undesirable. Stability also suffered because of the nature of the way in which the crystal accepts different regions of the source filament during rotation. This effect introduces fluctuations in the focused x-ray image geometry and intensity during the course of a measurement scan, thereby, degrading the resolution of the system.
Another deficiency of the prior art was the x-ray focusing crystal. The crystal was not in any way tailored to the present application. Therefore, the crystal was not designed to provide the most narrow and intense x-ray image as possible which is required to optimize resolution and scan frequency.